Re: [virtio-dev] Re: [PATCH v2 2/9] ALSA: virtio: add virtio sound driver

From: Takashi Iwai <tiwai@suse.de>
To: Anton Yakovlev <anton.yakovlev@opensynergy.com>
Cc: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>,
	<virtualization@lists.linux-foundation.org>,
	<alsa-devel@alsa-project.org>, <virtio-dev@lists.oasis-open.org>,
	<linux-kernel@vger.kernel.org>, Jaroslav Kysela <perex@perex.cz>,
	Takashi Iwai <tiwai@suse.com>,
	"Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [virtio-dev] Re: [PATCH v2 2/9] ALSA: virtio: add virtio sound driver
Date: Wed, 03 Feb 2021 18:39:59 +0100	[thread overview]
Message-ID: <s5hy2g582z4.wl-tiwai@suse.de> (raw)
In-Reply-To: <fb4808e7-28d6-996e-02fc-c63d0e1d8221@opensynergy.com>

On Wed, 03 Feb 2021 18:34:12 +0100,
Anton Yakovlev wrote:
> 
> Hi Takashi,
> 
> 
> On 03.02.2021 17:59, Takashi Iwai wrote:
> > On Tue, 02 Feb 2021 00:18:09 +0100,
> > Anton Yakovlev wrote:
> >>>> +/**
> >>>> + * virtsnd_reset_fn() - Kernel worker's function to reset the device.
> >>>> + * @work: Reset device work.
> >>>> + *
> >>>> + * Context: Process context.
> >>>> + */
> >>>> +static void virtsnd_reset_fn(struct work_struct *work)
> >>>> +{
> >>>> +     struct virtio_snd *snd =
> >>>> +             container_of(work, struct virtio_snd, reset_work);
> >>>> +     struct virtio_device *vdev = snd->vdev;
> >>>> +     struct device *dev = &vdev->dev;
> >>>> +     int rc;
> >>>> +
> >>>> +     dev_info(dev, "sound device needs reset\n");
> >>>> +
> >>>> +     /*
> >>>> +      * It seems that the only way to properly reset the device is to
> >>>> remove
> >>>> +      * and re-create the ALSA sound card device.
> >>>> +      *
> >>>> +      * Also resetting the device involves a number of steps with
> >>>> setting the
> >>>> +      * status bits described in the virtio specification. And the
> >>>> easiest
> >>>> +      * way to get everything right is to use the virtio bus interface.
> >>>> +      */
> >>>> +     rc = dev->bus->remove(dev);
> >>>> +     if (rc)
> >>>> +             dev_warn(dev, "bus->remove() failed: %d", rc);
> >>>> +
> >>>> +     rc = dev->bus->probe(dev);
> >>>> +     if (rc)
> >>>> +             dev_err(dev, "bus->probe() failed: %d", rc);
> >>>
> >>> This looks very suspicious to me. Wondering what ALSA maintainers
> >> will say
> >>> to this.
> >>
> >> I'm also wondering what the virtio people have to say. This part is a
> >> purely virtio specific thing. And since none of the existing virtio
> >> drivers processes the request to reset the device, it is not clear what
> >> is the best way to proceed here. For this reason, the most
> >> straightforward and simple solution was chosen.
> >
> > What is this "reset" actually supposed to do?  Reconfguring
> > everything, or changing only certain parameters, devices, whatever?
> 
> It means bringing this particular device to its initial state.
> 
> After that, the driver can re-read the configurations from the device
> and reconfigure everything.

So all running processes have to be finished before starting
resetting?  It sounds indeed like a complete device re-binding, and if
so, doing with the proper dev_*() API might be saner than the brute
force bus->remove() and bus->probe() calls...

thanks,

Takashi